Hi,
Sorry for the issues, I can help troubleshoot this with you.
Usually when getting the “Sorry, key validation server is not available.” is the plugin is blocked from connecting to our server, which often happens on development environments.
The license key is not needed for the plugin to work it is only needed for automatic updates.
You could wait to try and activate the license once the site is live.
Otherwise we have this article about development sites and using the license:
https://theeventscalendar.com/knowledgebase/using-one-license-for-your-live-and-dev-sites/
Let me know if that helps.
Thanks